In the field of medical diagnosis and graphic arts, there has been desired techniques relating to a photothermographic material in which efficient light-exposure is feasible as is done in a laser imager or laser image setter and by which definite, clear black images are obtained.
As a technique described above, a thermal imaging system employing organic silver salts is described, for example, in U.S. Pat. Nos. 3,152,904 and 3,487,075; D. Morgan, Dry Silver Photographic Material; and D. H. Klosterboer, “Thermally Processed Silver Systems” in IMAGING PROCESSES and MATERIALS, Neblette's Eighth Edition, edited by J. M. Sturge, V. Walworth, and A. Shepp (1969) page 279. Specifically, a silver salt photothermographic dry imaging material contains a reducible light-insensitive silver source (such as organic silver salts), a catalytically active amount of photocatalyst (such as silver halide) and a reducing agent, which are dispersed in a binder matrix. Such a photothermographic material is stable at ordinary temperature and, after exposure, form silver upon heating at a relatively high temperature (e.g., 80° C. or higher) through an oxidation reduction reaction between the reducible silver source (which functions as an oxidizing agent) and the reducing agent. The oxidation reduction reaction is accelerated by catalytic action of a latent image produced by the exposure. Silver formed through reaction of the reducible silver salt in exposed areas provides a black image, which contrasts with non-exposes areas, leading to image formation. The foregoing photothermographic material is also disclosed in literature, for example, U.S. Pat. No. 2,910,377 and JP-B No. 43-4924 (hereinafter, the term, JP-B refers to Japanese Patent Publication).
The silver salt photothermographic dry imaging material (which is hereinafter also denoted simply as photothermographic material) has often been used in medical diagnosis from its convenience. Fine representation is desired as medical diagnostic imaging so that high image quality with superior sharpness and enhanced graininess is required and blue black tone images tend to be favored in terms of easiness in diagnosis. However, it is rather difficult to produce neutral black image tone in such a photothermographic imaging system employing organic silver salts, so that image tone is modified using image toning agents, but such tone control is not sufficient to obtain an intended image and an improvement is still desired.
There was disclosed a technique to improve such a drawback, in which a specific reducing agent is used in combination with a specific compound, as described in JP-A No. 2002-169246 (hereinafter, the term, JP-A refers to Japanese Patent Application Publication). However, it was hard to say that images obtained in such a technique were a satisfactory image quality level for use in medical diagnosis. Thus, image color or density is easily changed by the action of light or heat during storage, producing relatively high fog density and low maximum density and resulting in serious inferior such that when exposed using a laser scanning exposure machine, the output image density significantly varies in response to a slight fluctuation in oscillation wavelength. Therefore, a further improvement is desired.